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Ok, so, basically, the rear cassette on my bike is giving me a lot of problems, and I'm not exactly a professional cyclist who would know where to begin so I thought I'd ask here;

The problem seems to sit with the chain going where chains shouldn't go. The other day when I was cycling along I chained gear for a hill or something, and there was a loud clunk and suddenly I could not move the pedals anymore. I got off my bike and had a look and the chain had gone beyond the casette and into the gap between the casette and the circular plastic gaurd that is up against the spokes.. It took litterally all of my strength to get it back out as it had wedged itself in there quite firmly.

I really don't know why that happened but I get the feeling something is wrong that needs fixing. I also have this odd behaviour with the rear casette, to go from gear 1 to 2, I have to twist the gear thing up to about halfway between 2 and 3, before i'll hear the chain move across, when left like this, I hear a scraping sound, and the way I stop that scraping sound is by twisting the gear thing back to just under 2. Perhaps something is misaligned?

I can only seem to use gear 1 through 4 on the front aswell but there are more gears it'll just never go into.

My bike gets knocked around alot and I guess it's abit abused, so could something have broken?

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    Clearly, you need to get the bike serviced. At a minimum the derailer needs to be adjusted, and it may be bent. Furthermore, it's fairly likely that the cables are sticking and need to be lubricated or replaced. You should be able to find good informatin on servicing the derailer on both Sheldon Brown's site and the Park Tool site. – Daniel R Hicks Apr 26 '15 at 12:10
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It is possible that you have damaged something but the most likely is that things need lube and are out of adjustment.

Check that the rear derailleur is vertical - if not go to a bike shop for help.

There are two screws (usually crosshead on the rear derailleur) that prevent the rear derailleur going beyond the ends of the cassette. The one that is for the big cog end needs adjusting so the chain cannot go beyond it into the plastic ring (I can't tell you which screw without looking on the web for the specific derailleur type).

The poor changing (front or rear) is probably due to the cable not moving well in its outer or cable stretch or poor adjustment. Try dripping some light lube into the end (eg thin Teflon, WD40) or if this is tricky undo the cable from the derailleur (note carefully which side of the screw it is on & remember this when replacing) and relube whole cable, checking for free movement when using the changer and pulling the end of the wire.

Re-attach the cable to the derailleur and use the adjustment nut on the derailleur to get the gears to index correctly (lots on the web about this: generally put the handlebar gear changer in the position for the smallest cog, pulling the wire taught manually & tighten the cable bolt. Then use the adjuster to get cable tightness exact for correct indexing).

If the cable condition is bad replace it and if possible the outers.

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You probably have a bent derailleur if the bike has been banged around and its not shifting right. You'll need to go to a bike shop so they can un-bend it, and then redo the derailleur adjustments and possibly cable tension. The bent derailleur will prevent the adjustments from taking place properly, so if you try them, you may find that its unadjustable.

As for the front, maybe you need to adjust the cable tensions or the front derailleur. Presumably you don't have a quad chainring setup since those are usually not found on bikes (sometimes trikes, though), but when you take the bike to the shop for the derailleur unbending (probably), you can have them tweak that adjustment too.

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